1. Field of the Invention
This invention relates to a method for removing volatile and semi-volatile contaminants from groundwater and soils in the subsurface of contaminated sites. More particularly, this invention relates to the use of biosparging and bioventing for removing volatile and semi-volatile contaminants from groundwater and soils in the subsurface of contaminated sites.
2. Description of Related Art
Biosparging and bioventing are commonly used to remove volatile and semi-volatile contaminants from groundwater and soils in the subsurface of contaminated sites. The conventional mode of treatment involves the introduction of air into the groundwater, a process referred to as biosparging, to partition the volatile/semi-volatile contaminants into the gas phase consistent with Henry's Law relationships, followed by the removal of the contaminants from the gas phase (air stream) through the biodegradative action of microbial films growing in the unsaturated zone of the soil, a process referred to as bioventing.
In many cases where biosparging is applied, the presence of iron in the groundwater as low as 10 mg per liter can cause serious plugging of the air injection system. This problem arises through the reaction of the oxygen in the air with the soluble iron in the water to form insoluble iron oxide precipitates in accordance with the following equations:2Fe2+O2=2FeO ppt2Fe3++1.5O2=Fe2O3 ppt
When groundwater iron is at levels above 10 ppm, substantial precipitates form at the natural ranges of groundwater pH (pH 6.5 to 8). These precipitates clog the injection points and pore spaces within the soil particles at locations proximal to the injection points. Plugging can occur within two weeks of start up of biosparging, causing airflows to be reduced by 50 to 90%, reducing the aeration zone of influence (for contaminant removal via air stripping) by more than 80% and causing system shutdown. This problem can potentially occur with any mode of in-situ biological treatment that involves the injection of substantial flows of air or oxygen into the groundwater. Examples of biological treatment modes where plugging problems can occur include aerobic heterotrophic treatment and methanotrophic treatment. Plugging can be severe to the extent that the biosparging/bioventing treatment for a site has to be abandoned for more costly treatment approaches, such as a groundwater pump-and-treat mode of remediation using a packed bed air stripping unit coupled to a granulated activated carbon adsorption process.
A secondary problem with conventional biosparging and bioventing treatment arises from the lack of independent control of the amount of oxidants and nutrients per pound of contaminant that are passed through the bioactive zone (the bioventing zone) of the unsaturated soil stratum. In conventional bioventing, the airflow is usually designed for the effective air stripping of the volatile and semi-volatile compounds from the groundwater. When treating concentrated groundwater contaminant source areas during the initial phase of air sparging, elevated concentrations of the contaminants may be transported to the gas phase and swept with the air stream into the bioventing zone, producing a situation where insufficient oxygen is available for the degradation of the contaminants.
A tertiary problem that occurs with conventional biosparging and bioventing treatments is that when air is used for stripping the groundwater, the oxygen stimulates aerobic bacteria in the groundwater to oxidize the hydrocarbon contaminants to form carbon dioxide and cell mass. When this happens, up to 0.5 pounds of cell mass per pound of contaminant oxidized may be formed, which can clog air injection points and the pores of the groundwater formation itself.
Similar conditions may occur with respect to the introduction of nutrient gases into the air stream used for contaminant stripping from the groundwater. If nutrient gases, such as nitrous oxide or triethylphosphate (TEP) are sparged with the air into the groundwater, most of the nutrients can be solubilized into the groundwater and not follow the air stream into the bioventing zone. In addition, with an initial phase of air sparging, the presence of high concentrations of hydrocarbons that are moved into the bioventing zone with the air may overwhelm the availability of nutrients required for biodegradation.